The present invention relates to a process for intermolecular etherification of mono- or polyhydric alcohols and for ether cleavage.
In a more particular aspect, the present invention relates to the treatment of the alcohol in the case of etherification or of the ether in the case of ether cleavage in an aqueous phase in the presence of an acid catalyst at a temperature of at least 100.degree. C. Even more particularly, the invention is directed at intermolecular etherification of diols, triols and tetrols with the formation of the corresponding di-, tetra- and hexahydroxyethers.
During the intermolecular etherification of alcohols in the presence of water and an acid catalyst, an equilibrium is established in a known way with the ether formed. Accordingly, ethers can also be formed and cleaved under the same reaction conditions in the process according to the invention. Etherification products of polyhydric alcohols, hereinafter known generally as polyhydroxyethers, are raw materials useful for various fields of application, including for the preparation of polyester resins, lubricants, PVC stabilizers and plasticizers.
Although intermolecular etherification is a basic reaction of organic chemistry, major problems often arise during the etherification of diols, triols, tetrols and other polyols due to insufficient selectivity towards the desired hydroxyether and due to the formation of polymers and colored impurities. In the case of etherification of 1,2-diols, the problem can be solved by using epoxides. In the case of etherification of diols with more than 2 carbon atoms between the hydroxyl groups, such as propane 1,3-diol or butane 1,4-diol for the purpose of obtaining monoetherification products, a hydroxyl function is usually protected before etherification is carried out--see J. Chem. Soc. Perkin Trans 1(1), (1992), 153-156. However, as a result, the etherification process becomes laborious and expensive.
The polyhydroxyethers of the di- and tripentaerythritol type (di- and tripenta) which are becoming increasingly important in industry, and the corresponding polyhydroxyethers of trimethylolethane (TME) and trimethylolpropane (TMP) can be obtained as coupling products in the well known preparation of penta, TME and TMP by way of an aldol and Cannizzaro reaction by varying the reaction parameters. The ether yields are mostly limited and always coupled with the preparation of the principal product penta, TME or TMP; see U.S. Pat. No. 2,441,597 and JP-A-8-176048.
According to JP-A-4-208242, pentaerythritol (penta) can be etherified in the melt in the presence of sulfuric acid with a dipenta selectivity of 35%, based on a penta conversion of 10 to 15%. A similar process for the preparation of dipenta from penta but in which the acid catalyst used is a phosphate of Ti, Al, Cr and Zr is known from EP-B 0 462 283.
Disadvantages of the process carried out in the absence of a solvent are the mostly large proportion of higher etherification products such as tri-, tetra- and polypenta and the formation of intensely colored impurities. A further disadvantage is the use of an organic solvent. As the conversion of penta has to be kept low in order to obtain a high dipenta selectivity, the labor involved in the work-up also increases. The process of the cited EP patent may also be carried out in the presence of water or an aprotic dipolar solvent: the selectivity towards dipenta may reach about 70% and the penta conversion 15% if sulfolane is used as solvent; a disadvantage, however, is the need to use an organic solvent.
About 8% dipenta and 3% tripenta are formed (JP-A 7-76541) by treating penta with urea (8 h, 190.degree. C.). If, in addition, a catalyst based on zirconium, titanium or tin and a dipolar aprotic solvent such as sulfolane are used, the dipenta yield may be increased to 20 to 25% (JP-A 7-258139, JP-A 7-188086 and JP-A 7-165653). Moreover, the etherification reaction mixture contains the catalyst in partially dissolved form, thereby making separation more difficult.
Alternative processes are based on the partial esterification of the hydroxyl functions of polyols, in order to suppress the production of oligomers or polymers in a downstream etherification (EP-B 0 550 611). The disadvantage of these reactions are the mediocre overall yields and the upstream esterification and the ester cleavage required afterwards in order to obtain the polyhydroxyether as a pure product. As a result of ester cleavage, large quantities of the corresponding carboxylic acid salts are obtained, which must be disposed of.
Whereas intramolecular etherification of sugar alcohols under hydrogenating conditions with H.sub.2 in the presence of a hydrogenation catalyst is well known (J. Mol. Cat. 91 (1994) 119-128), these conditions were never applied to intermolecular etherification. Moreover, the catalyst useful life proved to be limited and the selectivity mediocre.
An object of the present invention is, therefore, to obtain dialkylethers, di- and polyhydroxyethers by intermolecular etherification from the corresponding monomeric alcohols in good yield, but particularly with high selectivity with respect to the monoethers, and hereby largely to suppress the formation of higher oligomers, polymers, unsaturated by-products and polymeric secondary products thereof, and also discoloration.